MCQs on Power Systems

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01․ Making and breaking currents of 3 phase AC circuit breakers in power system are respectively in what form?
RMS and RMS value
Instantaneous and RMS value
RMS and Instantaneous value
any of the above

The instantaneous maximum or peak value of the current flowing through the circuit breaker at the instant of closing of circuit breaker is called making current. Making current = 2.55 * Breaking current The RMS value of short circuit fault current passing through the circuit breaker at the time of opening of circuit breaker is called breaking current

02․ In a 220 kV system, the inductance and capacitance up to the circuit breaker location are 25 mH and 0.025 µF respectively. The value of resistor required to be connected across the breaker contacts which will give no transient oscillations, is
25 Ω
250 Ω
500 Ω
1000 Ω

The value of resistor required to be connected across the breaker contacts which will give no transient oscillations R is R = 0.5* √(L/C) R = 0.5 * √(25*10-3/(0.025*10-6)) R = 500 Ω

03․ Which one of the following relays has the capability of anticipating the possible major fault in a transformer?
Over current relay
Differential relay
Buchholz relay
Over fluxing relay

Buchholz relay is the gas detector relay used to protect transformers which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by incipient breakdown of insulation.

04․ Mho relay is usually employed for protection of
short transmission lines only
long transmission lines only
medium transmission lines only
any line

Mho relays are usually employed for protection of long transmission lines only, as mho relay operation is not effected by power swings. Long transmission lines ----------- Mho relays Medium transmission lines ------- Impedance relays Short transmission lines ----------- Reactance relays

05․ Normally Z bus matrix is a
unit matrix
null matrix
full matrix
any of the above

Z bus algorithm or matrix is used for the fault analysis. Inverse of Y bus matrix gives the Z bus matrix, But Z matrix is a full matrix even though Y bus is a sparse matrix. More time is required for inverse of Z bus matrix if the size of the matrix is more than three.

06․ A generating station has a maximum demand of 100 MW, a load factor of 60% and a plant capacity factor of 50%. The reserve capacity of the plant is
10 MW
20 MW
80 MW
15 MW

Reserve capacity Rc = Plant capacity(Pc) - Maximum power(Pmax) Rc = Pmax*(load factor - capacity factor)/capacity factor Rc = 100*0.1/0.5 = 20 MW

07․ The rating of fuse wire is always expressed in

The current flowing through the fuse wire is same as the load current which is consumed by the electrical load. That load current only decides the rating of the fuse. Hence, the rating of the fuse is expressed in Amps.

08․ Lightning arresters are used for
protection of equipment from external lightning
protection of equipment from short circuits
protection of equipment from open circuits
all of the above

A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. For protecting the sub station equipment from the external lightning, lightning arresters are provided on both ends of the sub station. This is the first protecting device in the sub station.

09․ Lightning arresters are made up of which material?
silicon oxide
any of the above

Lightning arresters are made up of metal oxides like silicon oxide, zinc carbide which has non linear characteristics and it offers high resistance for normal supply voltage and low resistance for the lightning voltage. So that lightning is diverted to the earth.

10․ Which of the following transformer is used in sub station for operating a protective relay?
auto transformer
capacitor voltage transformer
distribution transformer
all of the above

A capacitor voltage transformer (CVT ), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.